I have searched far and wide and have not found a solution to get this achievement to track. I've put oil jars in sleeping guard tents and shot them with a fire bow and have had them die to the fire. I've used sleeping darts and placed oil jars next to them and then detonating them with a fire bow. They die to that fire and still no progress on the achievement. I've knocked guards out with unarmed and tried shooting a jar next to them and they still don't count. I've tried placing only 1 jar and placing many. Nothing works.